We present a psychophysical study of human arm movement imitation, and an approach to analyzing the resulting data, which can be applied to human or humanoid movement analysis. We describe a joint-space based segmentation and comparison algorithm that allow us to evaluate the performance of 11 different subjects on a series of arm movement imitation tasks. The results provide analytical evidence for the strong interference effects of simultaneous rehearsal during observation. Additionally, the results also demonstrate that repeated imitation in these tasks did not affect the subjects' performance.